Acquired immune deficiency syndrome (AIDS) is a serious disease of major public health importance. Immune electron microscopy, which is the direct observation of antigen-antibody interaction, has been employed as a means for detecting the fastidious etiologic agents of several diseases such as the 27 nm Norwalk agent of acute nonbacterial gastroenteritis, and the 27 nm hepatitis A virus of hepatitis. Collaborative studies using electron microscopy were carried out on simian AIDS specimens in collaboration with the NINCDS. A retrovirus-like agent was visualized by negative stain electron microscopy.